A mobile communication system uses a reference signal (RS) for channel estimation, channel quality estimation, measurement of a reception level for controlling transmission power and so forth. 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) adopts a Zadoff-Chu (ZC) sequence as RS used for an uplink and a downlink. The ZC sequence is adopted as RS because its frequency characteristic is uniform and its auto-correlation characteristic and cross-correlation characteristic are excellent. The ZC sequence is a kind of a GCL (Generalized Chirp-Like) sequence and is expressed by following equation 1.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                                x              u                        ⁡                          (              n              )                                =                      ⅇ                                          -                j                            ⁢                                                          ⁢                                                π                  ⁢                                                                          ⁢                  u                  ⁢                                      {                                                                  n                        ⁡                                                  (                                                      n                            +                            1                                                    )                                                                    +                      qn                                        }                                                                    N                  ZC                                                                    ,                                  ⁢                  0          ≤          n          ≤                                    N              ZC                        -            1                                              [        1        ]            
In equation 1, NZC is the sequence length of the ZC sequence, u is the ZC sequence number, and NZC and u are integers, which are “prime” to one another. Furthermore, n is the symbol number of the ZC sequence and q is an arbitrary integer. Furthermore, 3GPP LTE is studying use of a ZC sequence expressed by equation 2, which assumes q=0 in equation 1.
                    (                  Equation          ⁢                                          ⁢          2                )                                                                                                x              u                        ⁡                          (              n              )                                =                      ⅇ                                          -                j                            ⁢                                                          ⁢                                                π                  ⁢                                                                          ⁢                  un                  ⁢                                      (                                          n                      +                      1                                        )                                                                    N                                      ZC                    ⁢                                                                                                                                                    ,                                  ⁢                  0          ≤          n          ≤                                    N              ZC                        -            1                                              [        2        ]            
Furthermore, the ZC sequence allows NZC−1 quasi-orthogonal sequences with excellent cross-correlation characteristics to be generated from the ZC sequence whose sequence length NZC is a prime number.
Furthermore, RS used on an uplink according to 3GPP LTE is transmitted in a transmission bandwidth of an integer multiple of 1 RB (Resource Block) composed of 12 subcarriers (e.g., see Non-Patent Document 1). Thus, the sequence length of the ZC sequence, which is a prime number, does not match the number of subcarriers, which corresponds to the transmission bandwidth of RS. For example, for RS transmitted with 1 RB (12 subcarriers), a ZC sequence whose sequence length NZC is 11 or 13 is used, and for RS transmitted with 2 RBs (24 subcarriers), a ZC sequence whose sequence length NZC is 23 or 29 is used.
Here, as the method of obtaining a ZC sequence having a sequence length corresponding to the number of subcarriers equivalent to the transmission bandwidth of RS, two methods are under study; “truncation” that deletes part of the ZC sequence and “extension” that extends part of the ZC sequence (e.g., see Patent Document 1). Of symbols constituting the ZC sequence whose sequence length is greater than the number of subcarriers equivalent to the transmission bandwidth of RS, “truncation” deletes symbols corresponding to the portion that exceeds the number of subcarriers. On the other hand, “extension” extends by adding symbols corresponding to the shortage relative to the number of subcarriers to symbols constituting the ZC sequence whose sequence length is smaller than the number of subcarriers corresponding to the transmission bandwidth of RS. When, for example, a ZC sequence whose sequence length N is 13 is used, RS of 12 symbols is generated by truncating the ZC sequence, that is, by deleting the last one symbol of the ZC sequence. On the other hand, when a ZC sequence whose sequence length N is 11 is used, RS of 12 symbols is generated by extending the ZC sequence, that is, by adding a copy of the first one symbol of the ZC sequence to the end thereof (e.g., see Non-Patent Document 2).
Furthermore, 3GPP LTE is also studying use of a CS (Cyclic-Shifted)-ZC sequence, which is a cyclically shifted ZC sequence obtained by truncation or extension as RS. The CS-ZC sequence is a sequence generated by making one ZC sequence cyclically shift by an amount of cyclic shift, which differs from one cell to another or from one radio communication apparatus on a transmitting side to another. The radio communication apparatus in the receiving side performs correlation operation of the received RS, detects each RS within a detection range preset for each cell according to the amount of cyclic shift and thereby separates different cells or different mobile units. Unless a propagation delay time of a radio transmission terminal exceeds the amount of shift, cross-correlation between sequences becomes 0 and the CS-ZC sequences are orthogonal to each other, and therefore using the CS-ZC sequence as RS increases the detection accuracy of RS.    Patent Document 1: US Patent Application Laid-Open No. 2005/0226140 Specification    Non-Patent Document 1: TS36.211 v1.2.0, “3GPP TSG RAN; Physical Channels and Modulation (Release 8)”    Non-Patent Document 2: Fujitsu, R1-070365, “Structure of uplink reference signal”, 3GPP TSG RAN WG1 #47bis Meeting, Sorrento, Italy, Jan. 15-19, 2007